Borderlands and Demon's Souls were two of 2009's biggest hits, measured by sales and/or popular acclaim. So there's a nice symmetry in seeing Borderlands 2 paying homage to Dark Souls with an Easter egg encountered in the Caustic Caverns.

There's a ghost sitting on a rock, which is enough of a visual Easter egg. But the snickering NPC's name is Solitaire, which might reference the insane Solaire of Astora, a fan-favorite character in Dark Souls.

YouTube user Rofl316 uploaded this video, saying he waited at the campsite for 30 minutes to see if there was a puzzle or a quest associated. Near as he can tell, there is not.

Dark Souls Makes An Appearance In Borderlands 2 [Game Informer. YouTube video uploaded by Rofl316]